Porous-plug low work-function film cathodes for electron-bombardment ion thrustors



Mafch 5, 1968 w. o. ECKHARDT 3,371,489

POROUS-PLUG LOW WORK-FUNCTION FILM CATHODES FOR ELECTRON-BOMBARDMENT ION THRUSTORS Flled Oct. 23, 1964 2 Sheets-Sheet 1 1o DISCHARGE CATHODE HEATERS o DISCHARGE CHAMBER ANODE EXPELLANT 5 5 VALVE ION ACCELERATOR )4) 0 E H I EXTRACTION EL TROD SYSTEM SCREEN ELECTRODE POROUS PLUG (CATHODE SUBSTRATE) DISCHARGE CHAMBER END PLATE 1 I T B MAGNETIC FIELD BOILER HEATER O 0 \TJ T0 ACCELERATOR POWER SUPPLY.

Wilfried O.-Eckhord1,

INVENTOR.

BY. M

ATTORNEY.

I Filed om. 2:5, 1964 March 5, 1968 w. o. ECKHARDT 3,371,489

POROUS-PLUG LOW WORK-FUNCTION FILM CATHODES FOR ELECTRON-BOMBARDMENT ION THRUSTORS 2 Sheets-Sheet 2 BOILER I non MAGNET |I I| EXTRACTION COILS SYSTEM DISCHARGE CHAMBER ANODE Wilfried O. Eckhordt,

INVENTOR.

grit-210.5%

ATTORNEY.

United States Patent Ofifice 3 371,489 PGROUS-PLUG LGVi WORK-FUNCTIUN FILM CATHODES FOR ELECTRON BOMBARD- MENT IUN THRUSTORS Wilfried O. Eckhardt, Malibu, Calif, assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Get. 23, 1964, Ser. No. 405,078 7 Claims. (Cl. 60-202) ABSTRACT OF THE DESCLOSURE A cathode for use in an electron bombardment ion engine comprising a porous plug of tungsten and means for feeding cesium therethrough to produce a thin film of cesium thereon. The plug is heated sufficiently to thermionically expel electrons therefrom and to evaporate cesium atoms therefrom but not suflicient to expel cesium ions therefrom. This porous plug thus acts both as the source of ionizing electrons and as the supply for neutral propellant atoms.

This invention relates to a method and apparatus for improving the power efficiency and the cathode l1fe of electron-bombardment ion sources employed in 1011 thrustors. Such thrustors have been described by H. R. Kaufman in NASA Technical Note D585, January 1960, entitled, An Ion Rocket With an Electron-Bombardment Ion Source.

Previous electron bombardment ion thrustors suffered from the disadvantage of short cathode life-times due to sputtering caused by misdirected positively charged ions bombarding the cathode filament which is at a relatively negative potential. In the present invention the tungsten cathode is covered with a layer of cesium. The bombarding ions only hit the temporary cesium layer not the permanent tungsten cathode, thus providing very long cath-. ode life-times.

Previous contact or surface ionization type ion engines, which use cesium-covered tungsten surfaces heated sufficiently high to expel cesium ions therefrom, suffer from the disadvantage of high power loss due to thermal radiation from the high temperature tungsten. The thermionic cathode of this invention superficially resembles such porous plug ion sources. The primary difference between the two is that the tungsten plug of this invention is held at a much lower temperature than the tungsten plug of such known devices. The temperature of the plug of this invention is not high enough to expel cesium ions therefrom but only sufiicient to expel electrons and to evaporate cesium atoms therefrom.

It is an object of this invention to admit a low workfunction expellant into the discharge chamber of an electron-bombardment ion thrustor by feeding it through a porous plug consisting of a high work-function material, whereby a thin film of the expellant material gets adsorbed on the surface of the porous plug. This film-covered plug is used as the thermionic cathode of the discharge by heating it to that temperature which, at the required expellant flow rate, yields that electron emission current which maximizes the product of expellant mass utilization efiiciency (this is the standard term used in the field of electric propulsion to tell what fraction of the 3,371,489 Patented Mar. 5, 1968 expellant is actually used to produce thrust; in an electric propulsion engine; this term is equal to the ion beam current divided by the expellant atom flow rate times the electron charge) times thrustor power etficiency (the ratio of the power in the jet to the power supplied to the thrustor; this might also be referred to as the conversion efficiency occurring in the thrustor).

The following mechanism makes this mode of operation possible: It is Well known that materials possessing low work functions in the bulk can be made to exhibit even lower work functions when adsorbed as a film of close to one monolayer thickness on a substrate with a high work function. The film thickness giving the lowest work function possible for a given combination of film material and substrate material depends on the choice of these two materials, but is usually between one-half and one monolayer. The work functions obtainable by this technique are so low that electron emission current densities of the order of several amperes per square centimeter can be obtainedat cathode temperatures at or below 1000" K., thus resulting in very efficient cathode operation. Applications of this technique, however, are limited to cases where evaporation of the film material from the substrate is not objectionable.

According to this invention, in an electron-bombardment ion thrustor, the evaporation of the film material from the porous plug substrate is utilized as the mechanism for feeding the expellant into the discharge chamber. Since the materials exhibiting the lowest work functions also have low ionization potentials, the choice of the expellant according to work function optimization considerations leads simultaneously to an expellant which can be ionized in a low-voltage discharge. More specifically, the discharge voltage can be kept below the sputtering threshold of the porous plug cathode substrate material, thus resulting in very long cathode life.

In order to achieve an expellant mass utilization sufficient for practical thrustor applications, the porous plug cathodes should provide an admission ratio, defined as number flux of electrons (or the number of electrons) into the discharge chamber divided by number flux of expellant atoms (or the number of expellant atoms) into the discharge chamber, which is at least of the order of ten, and preferably higher. Examples of materials suitable for obtaining the required admission ratio are: cesium as the expellant, and tungsten, nickel, or platinum as the porous plug cathode substrate. Nickel and platinum should be capable of producing higher admission ratios than tungsten because of their higher work functions. These examples should in no way be considered exclusive.

FIG. 1 shows, as a specific example for many possible embodiments of this invention, a schematic cross section through an ion thrustor using a porous-plug low workfunction film cathode of the type described herein.

FIG. 2 is a cutaway drawing of an experimental ion thrustor with a 6 cm. anode diameter, which has been used successfully to test the porousplug low work-function film cathode concept of this invention.

The electron-bombardment type ion thrustor of FIGS. 1 and 2 ionizes the expellant by electron bombardment in the discharge chamber. An axial magnetic field is employed to prevent the rapid escape of electrons to the anode. Some of the ions thus created arrive at the screen electrode and are accelerated out by the potential field between the screen and the accelerator electrode.

3 Table I gives typical results obtained in actual operation of an electron-bombardment ion thrustor in accordance with the invention:

TABLE I Operating parameters for 6-cm. diameter electronbombardment ion thrustor with porous-plug work-function film cathode.

Expellant Cesium.

Cathode substrate Porous nickel plug. Cathode area 2 cm.

Cathode temperature 910 to 1008 K. Discharge current 2.5 to a. Discharge voltage 8 to 10 v.

Magnetic field 20 to 40 G. Maximum ion beam current 200 ma.

The work function of the surfaces of a solid or liquid is defined as the average energy required to displace an electron from the material to the vacuum outside the surface of the material.

Obviously many other modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention can be practiced otherwise than as specifically described.

What is claimed is:

1. An electron source comprising:

a cathode made of a relatively high work-function material;

means providing a supply of relatively low Workfunction material;

means for applying and maintaining a coating of approximately a monolayer thickness of said low workfunction material on said cathode; and

means for maintaining the temperature of said cathode sufiiciently high to thermionically emit electrons therefrom but below the temperatures required to expel ions of said low work-function material.

2. The apparatus according to claim 1 in which said high work-function material is tungsten, nickel, or platinum and wherein said low work-function material is cesium.

3. In an electron bombardment ion thrustor of the type including an engine chamber, a propellant supply means upstream of the engine chamber for feeding neutral atoms of gaseous propellant into said chamber, an cathode situated in said chamber for supplying ionizing electrons for ionizing said gaseous propellant, an anode situated within said chamber and maintained at a positive potential relative to said cathode, magnetic coils encircling said engine chamber for creating axial magnetic field lines about which said electrons gyrate until they collide with said atoms which causes the electrons to drift to said anode, the improvement comprising:

a porous plug made of a high work-function material mounted on said ion thrustor to provide a surface facing said chamber;

means for feeding a low work-function propellant into a side of said plug opposite said surface such that said surface becomes covered by a thin film of said propellant; and

means for maintaining the temperature of said plug at a temperature such that electrons are thermionically emitted therefrom and neutral atoms of said propellant are evaporated therefrom, but which temperature is below that required to emit ionized propellant therefrom, whereby said porous plug is both said propellant supply means and said cathode.

4. The apparatus according to claim 3 in which said material is tungsten, nickel, or platinum and said propellant is cesium.

5. The method of producing electrons comprising:

providing a cathode substrate made of a relatively high work-function material;

applying and maintaining a coating on said substrate of approximately a monolayer thickness of a material of relatively low work-function; and

heating said substrate to a temperature sufiicient to expel electrons therefrom but less than the temperature required to expel ions of said low work-function material therefrom.

6. The method according to claim 5 in which said high work-function material is tungsten and said low workfunction material is tungsten and said low work-function material is cesium.

7. The new use of a cesium-covered tungsten surface in an ion beam generator as both a source of neutral atoms and as a cathode source of ionizing electrons comprising the steps of maintaining the temperature of said tungsten surface sutficient to thermionically expel electrons therefrom but below the temperature required to expel. cesium ions therefrom; and

applying a potential difference between said cathode source and an external anode to attract electrons from said cathode source.

References Cited UNITED STATES PATENTS 3,014,154 12/1961 Ehlers et a1. 35.5 3,117,416 1/1964 Harries 60-355 3,156,090 11/1964 Kaufman 6035.5 3,210,926 10/1965 Forbes et al. 6035.5 3,233,404 2/1966 Huber et al. 6035.5

OTHER REFERENCES Electrical Engineering, July 1963 (pp. 459, 460 and 461 relied on).

Stuhlinger, Ernest, Ion Propulsion for Space Flight, McGraw-Hill, New York, June 15, 1964, (pp. 182-185 and 193-199 relied on).

CARLTON R. CROYLE, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,371,489 March 5, 1968 Wilfried O. Eckhardt It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column I6, line 7, after "porous-plug" insert 10w colunm 4, lines 26 and 27, strike out "and said low workfunctlon material is tungsten".

Signed and sealed this 24th day of June 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR. 

